<?php

/**
 * This function solves the problem 8 of the Project Euler.
 *
 * Problem description:
 *The four adjacent digits in the 1000-digit number that have the greatest product are 9 × 9 × 8 × 9 = 5832.

 *73167176531330624919225119674426574742355349194934
 96983520312774506326239578318016984801869478851843
 85861560789112949495459501737958331952853208805511
 12540698747158523863050715693290963295227443043557
 66896648950445244523161731856403098711121722383113
 62229893423380308135336276614282806444486645238749
 30358907296290491560440772390713810515859307960866
 70172427121883998797908792274921901699720888093776
 65727333001053367881220235421809751254540594752243
 52584907711670556013604839586446706324415722155397
 53697817977846174064955149290862569321978468622482
 83972241375657056057490261407972968652414535100474
 82166370484403199890008895243450658541227588666881
 16427171479924442928230863465674813919123162824586
 17866458359124566529476545682848912883142607690042
 24219022671055626321111109370544217506941658960408
 07198403850962455444362981230987879927244284909188
 84580156166097919133875499200524063689912560717606
 05886116467109405077541002256983155200055935729725
 71636269561882670428252483600823257530420752963450

 *Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?
 */

/**
 * @return int
 */
function problem8(): int
{
    $theNumber = "73167176531330624919225119674426574742355349194934" .
        "96983520312774506326239578318016984801869478851843" .
        "85861560789112949495459501737958331952853208805511" .
        "12540698747158523863050715693290963295227443043557" .
        "66896648950445244523161731856403098711121722383113" .
        "62229893423380308135336276614282806444486645238749" .
        "30358907296290491560440772390713810515859307960866" .
        "70172427121883998797908792274921901699720888093776" .
        "65727333001053367881220235421809751254540594752243" .
        "52584907711670556013604839586446706324415722155397" .
        "53697817977846174064955149290862569321978468622482" .
        "83972241375657056057490261407972968652414535100474" .
        "82166370484403199890008895243450658541227588666881" .
        "16427171479924442928230863465674813919123162824586" .
        "17866458359124566529476545682848912883142607690042" .
        "24219022671055626321111109370544217506941658960408" .
        "07198403850962455444362981230987879927244284909188" .
        "84580156166097919133875499200524063689912560717606" .
        "05886116467109405077541002256983155200055935729725" .
        "71636269561882670428252483600823257530420752963450";
    $greatestProduct = 0;
    $substringSize = 13;

    for ($i = 0; $i < strlen($theNumber) - $substringSize; $i++) {
        $currentSubstring = substr($theNumber, $i, $substringSize);
        $currentProduct = 0;

        for ($j = 0; $j < strlen($currentSubstring); $j++) {
            $currentProduct = ($currentProduct == 0 ? (int)$currentSubstring[$j] : $currentProduct * (int)$currentSubstring[$j]);
        }

        $greatestProduct = ($greatestProduct < $currentProduct ? $currentProduct : $greatestProduct);
    }

    return $greatestProduct;
}
